Roman Numeral Converter
Convert numbers to Roman numerals and Roman numerals back to numbers, with validation.
Roman numeral
MCMXCIV
Updates live as you type
Frequently asked questions
Letters represent values: I=1, V=5, X=10, L=50, C=100, D=500, M=1000. They are added left to right, except when a smaller value precedes a larger one (IV=4, IX=9), where it is subtracted.
Standard Roman numerals go up to 3999 (MMMCMXCIX). Larger numbers need overline notation, which this converter does not use.
Those are non-standard. The converter validates by re-encoding the value, so it flags malformed numerals instead of silently returning a wrong number.
Clock faces, book chapters, movie sequels, monarch and pope names, and the year in film credits and monuments.
MCMXCIV isn't guessed — it's rebuilt from four subtractive rules at once
1994 breaks down as M (1000) + CM (900) + XC (90) + IV (4), giving MCMXCIV — a value that happens to exercise every subtractive rule Roman numerals have in a single number, which is exactly why it shows up so often as the go-to example. Run the conversion in reverse on "MCMXCIV," and it correctly recovers 1994, confirming the two directions are genuine inverses of each other rather than two separately-coded approximations that happen to usually agree.
Why a malformed input gets rejected instead of guessed at
Feed in a non-standard numeral like "MDCCCCLXXXXIIII" — a valid-looking but malformed stand-in for 1994 that doesn't follow proper subtractive notation — and the validator rejects it outright instead of silently returning a wrong number. It does this by re-encoding whatever value it computes and checking that the result matches a well-formed numeral; anything that doesn't match, including non-standard forms like IIII or VV, gets flagged rather than guessed at. A wrong silent conversion would be a much worse failure than an honest rejection.
The mechanical rule underneath both directions
The standard symbols — I, V, X, L, C, D, M — combine using subtractive notation, where a smaller symbol placed before a larger one is subtracted rather than added: IV is 4, IX is 9, XL is 40, CM is 900. Both conversion directions apply this same rule set, just running it forward or backward depending on the mode selected.
Standard Roman numerals cover 1 to 3999; anything beyond that needs overline notation outside this tool's scope, so out-of-range numbers get flagged too rather than silently truncated. Use this for homework, design work, tattoos, dates and decoding inscriptions on clocks, monuments or old documents, with the confidence that a malformed or out-of-range input will be caught rather than mishandled.